A ``shelf'' is a persistent, dictionary-like object. The difference
with ``dbm'' databases is that the values (not the keys!) in a shelf
can be essentially arbitrary Python objects -- anything that the
pickle module can handle. This includes most class
instances, recursive data types, and objects containing lots of shared
sub-objects. The keys are ordinary strings.

To summarize the interface (key is a string, data is an
arbitrary object):

The choice of which database package will be used
(e.g. dbm or gdbm) depends on which interface
is available. Therefore it is not safe to open the database directly
using dbm. The database is also (unfortunately) subject
to the limitations of dbm, if it is used -- this means
that (the pickled representation of) the objects stored in the
database should be fairly small, and in rare cases key collisions may
cause the database to refuse updates.

Dependent on the implementation, closing a persistent dictionary may
or may not be necessary to flush changes to disk.

The shelve module does not support concurrent read/write
access to shelved objects. (Multiple simultaneous read accesses are
safe.) When a program has a shelf open for writing, no other program
should have it open for reading or writing. Unix file locking can
be used to solve this, but this differs across Unix versions and
requires knowledge about the database implementation used.